Dembele is 28, he started his career as a striker, he was signed by Fulham to operate as a winger/striker. We signed him to play central midfield. He was then moved to various positions, with the debate on here (when Sherwood was manager) moving towards Dembele offering more in attack and goal return, something which was never going to happen.
Regardless of his natural ability, Dembele will never be consistent, to a level that defines him as a player in a set role.
On his day, he can look world class, but any excuses made for his inconsistent form are just that, as sometimes players are inconsistent and frustrate fans/manager, and Dembele is one of them players.
Maybe if he had been coached properly at a much younger age and found his best role, his career would have been different, but this is where Dembele is and he's nothing more then a squad player that will occasionally have a superb game.